The present invention relates to a harmonic gear drive, which is compact in size and light-weighted, and which can realize high positional accuracy and high gear reduction ratios. More particularly, the present invention relates to a harmonic gear drive, which is suitable for applications in the field of automated machinery such as an articulation driving portion for a robot where silence and smoothness of driving are required, a semiconductor manufacturing device, and reduction gears for a precision actuator, and in the field of aerospace technology such as a control mechanism and a driving mechanism.
Harmonic gear drive is a mechanical reduction gear device for the transmission of power, which is light and compact and can realize high positional accuracy and high gear reduction ratios without employing any complicated mechanism and structure and with free of backlash. The harmonic gear drive is known as a mechanical reduction gear device including a flexspline (flexible gear).
Typically, the conventional harmonic gear drive consists of a rigid circular outer spline with internal teeth, a flexspline with external teeth partly meshed with the internal teeth of the circular spline in a flexible meshing manner, and a wave generator for displacing the meshing points of those internal teeth and external teeth in the circumferential direction for the different number of teeth by means of elastic flexibility. This type of conventional harmonic gear drive is disclosed, for example, in Japanese Laid-open Patent Application No. 10-110790 (paragraphs [0002] to [0003]; FIGS. 1 and 2), which is also referred to as Patent reference 1.
As seen in FIGS. 4 and 5, the harmonic gear drive 100 disclosed in the above Patent reference 1 comprises a rigid circular outer spline 200 with internal teeth, a flexspline 300 positioned inside the circular spline 200 and including external teeth, and an oval-shaped wave generator 400 fitted into the flexspline 300 through a bearing. The flexspline 300 includes a cylindrical portion 310, an annular diaphragm portion 320 continuously extending from the proximal side of the cylindrical portion 310, a boss (attachment portion) 330 formed in the center of and integrally with the diaphragm portion 320, and external teeth 340 formed on the outer peripheral surface of an open end portion which is positioned at the distal end of the cylindrical portion 310.
When the wave generator 400 is rotated by the rotation of the motor rotation shaft of the drive source, the flexspline 300 undergoes elastic deformation together with the outer race of the bearing which allows elastic deformation, so that the meshing points between the external teeth 340 of the flexspline 300 and the internal teeth of the rigid circular outer spline 200 displace in the circumferential direction. In general, since the rigid circular outer spline 200 is fixed to a stationary member, a rotation force is output from the boss (attachment portion) 330 of the flexspline 300 while the speed is greatly reduced in accordance with the difference of the number of teeth between the external teeth and the internal teeth.
In the flexspline 300, the cylindrical portion 310 which allows elastic deformation and the annular diaphragm portion 320 are joined together via a bent portion extending from the proximal end of the cylindrical portion 310 in the direction radially and inward toward the center of the boss 330. The flexspline 300 includes the annular boss (attachment portion) 330 integral with and positioned at the inner periphery of the diaphragm portion 320, and the external teeth 340 formed on the outer peripheral surface of the open end portion which is positioned at the distal end of the cylindrical portion 310.
This conventional harmonic gear drive 100 is light-weighted and has no backlash when compared with other reduction gear drives such as an involute planetary gear drive, and a cycloid planetary gear drive, and therefore it is suitable for applications where high positional accuracy is required. However, this harmonic gear drive has a drawback that it is difficult to reduce vibration generated from the meshing mechanism inside the gear drive.
In order to improve the aforementioned drawback concerning vibration characteristic of the harmonic gear drive 100 disclosed in Patent reference 1, a flat type harmonic gear drive is proposed in which an axial length of the cylindrical portion 310 except for the part where the external teeth 340 are provided is set smaller than the length of the diaphragm portion 320 in the radial direction. This flat type harmonic gear drive is disclosed in Domestic re-publication of WO98/53224 (pages 6-7; FIG. 1), which is also referred to as Patent reference 2.
The flay type harmonic gear drive disclosed in Patent reference 2 is a flat-shaped harmonic gear drive with large outside dimension and small axial length. According to this flat type harmonic gear drive, it is possible to decrease an occurrence of vibration while maintaining advantages of the harmonic gear drive such as improvements in rigidity, damping capacity and gear accuracy, and lightness and compactness in size.
However, the harmonic gear drive 100 as disclosed in Patent reference 1 has a drawback that since the meshing between the rigid circular outer spline 200 and the flexspline 300 is performed by the rotation of the wave generator 400, it is difficult to restrict an occurrence of high-pitched noise caused by the meshing mechanism.
In the harmonic gear as disclosed in Patent reference 2, the drawback concerning the vibration characteristic of the harmonic gear drive 100 according to Patent reference 1 may be improved. However, since rigidity and damping capacity of the harmonic gear drive 100 are maintained at high level, a problem for noise reduction at a drive part still remains, for instance, on application of robots where silence and flexibility are required.
In order to provide a flexible meshing gear of the harmonic gear drive, a thin-walled flexible portion of the flexspline which allows elastic deformation has to be thin-walled with high-strength. For this reason, high responsive control to the components of the harmonic gear drive results in a vibration inducing source so that the thin-walled flexible portion vibrates and a resonance phenomenon is caused in the cylindrical portion. Therefore, as described above in the prior art, it is difficult to solve the drawbacks both in vibration and noise.
When the harmonic gear drive is applied for a robot, resonance occurs at the cylindrical portion of the thin-walled portion of the driven unit due to frequency responsive to the rotation speed of the motor that is connected to the wave generator, which leads to a noise problem. As a result, the harmonic gear drive fails to achieve silent driving and affects fatigue of the components, which makes it worse to realize the merchantability. For instance, if the characteristic value of the vibration frequency for the flexspline is more or less 200 Hz, the motor rotation speed of the drive source is 6000 rpm (100 Hz), and the control responsive frequency is 1 KHz, resonance is liable to occur.
In view of the above, the present invention seeks to provide a harmonic gear drive which can reduce vibration and noise caused by meshing between the components which consist of the meshing mechanism of the harmonic gear drive.